Methods of signal substitution for maintenance of amplifier saturation

ABSTRACT

A method for maintaining amplifier saturation in a wavelength division multiplexed (WDM) optical network having a plurality of sub-bands, each sub-band including at least two signal channels which carry respective data signals, and a plurality of substitute signal transmitters, each substitute signal transmitter generating a substitute signal and corresponding to a respective one of the plurality of sub-bands, includes identifying signal channels having a predetermined characteristic within each of the plurality of sub-bands. A substitute signal transmitter is turned on if the sub-band corresponding to the substitute signal transmitter includes a predetermined number of signal channels having said predetermined characteristic. The data signals and the substitute signals are combined into a WDM signal, and the WDM signal is transmitted over an optical transmission fiber.

FIELD OF THE INVENTION

The present invention relates generally to optical communications, andmore particularly to a method for maintaining amplifier saturation in awavelength division multiplexed (WDM) optical communication system.

BACKGROUND OF THE INVENTION

In a dense wavelength division multiplexed (DWDM) system having a highnumber of signal channels, the initially deployed system typically doesnot have all of the signal channels fully populated. The number ofsignal channels that are populated depends upon several factorsincluding the amount of capacity initially required in the DWDM system.To make the amplifiers in the system operational during the lifetime ofthe system, from initial deployment at less than capacity to fullcapacity after upgrades, a substitute signal may be used to fill one ormore empty channels to maintain the saturation (and other) performancecharacteristics of the amplifier. In conventional narrowband Erbiumdoped fiber amplifier DWDM systems, it is theoretically possible for asfew as one substitute signal to be used to saturate the amplifier,depending on the operational bandwidth of the substitute signal,although commercial applications employ a substitute signal for eachchannel.

Raman amplified systems use numerous pump wavelengths to achieve a muchlarger operational bandwidth than an Erbium doped fiber amplifiedsystem. The saturation mechanism for the Raman amplifier is thedepletion of the individual pumps. This depletion occurs throughpump-pump and pump-signal interactions. As a result, a more delicatebalance of substitute signals is used to maintain the system performanceof the Raman amplifier. In conventional Raman-amplified systems, asubstitute signal is used for each unused signal channel.

As capacity demands continue to grow, efforts are focusing on increasingthe usable bandwidth in both EDFA and Raman optical communicationsystems. As the number of channels increases, the cost associated withproviding a substitute signal laser for each channel rises in tandem.Accordingly, it would be desirable to provide systems and methods formaintaining amplifier saturation (and system performance) at reasonableprices as the available bandwidth increases.

SUMMARY OF THE INVENTION

Briefly, in one aspect of the invention, a method for maintainingamplifier saturation in a wavelength division multiplexed (WDM) opticalnetwork having a plurality of sub-bands, each sub-band including atleast two signal channels which carry respective data signals, and aplurality of substitute signal transmitters, each substitute signaltransmitter generating a substitute signal and corresponding to arespective one of the plurality of sub-bands, includes identifyingsignal channels having a predetermined characteristic within each of theplurality of sub-bands. A substitute signal transmitter is turned on ifthe sub-band corresponding to the substitute signal transmitter includesa predetermined number of signal channels having said predeterminedcharacteristic. The data signals and the substitute signals are combinedinto a WDM signal, and the WDM signal is transmitted over an opticaltransmission fiber.

In another aspect of the invention, the predetermined characteristic isone or both of unused and inoperable.

In yet another aspect of the invention, a signal channel is determinedto have the predetermined characteristic based on the detectedwavelengths and power levels of the data signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a block diagram of an exemplary terminal unit consistentwith the present invention.

FIG. 1(b) is a block diagram of a substitute signal architectureconsistent with the present invention.

FIG. 2 is a block diagram of a combining circuit consistent with thepresent invention.

FIG. 3 is a flow diagram for processing a system command in thearchitecture of FIG. 1.

FIG. 4 is a flow diagram for identifying and compensating for failedsignal channels in the architecture of FIG. 1.

FIG. 5 is a flow diagram for compensating for power and wavelength driftin the architecture of FIG. 1.

FIG. 6 is a block diagram of a redundancy circuit consistent with thepresent invention.

FIGS. 7(a)-10(b) are graphs which depict transmitted and receivedsignals for various combinations of data signals and substitute signalsaccording to the exemplary embodiment of the present invention.

FIG. 11 is a flow diagram for switching channels from a service fiber toa protect fiber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1(a) is a block diagram of the transmit portion of a terminal unitincluding substitute signal architectures consistent with the presentinvention. Therein, a plurality of long reach transceivers (LRTRs) 2each generate the optically modulated data signals to be transmitted bythe terminal unit. These wavelength channels are wavelength divisionmultiplexed by unit 4 to form a composite signal. Substitute signals arecoupled to the multiplexed signal from substitute signal architecture10, prior to being amplified for transmission by amplifier 12 andtransmitted over, e.g., submarine or terrestrial cable 14. Those skilledin the art will appreciate that the multiplexing of data signals can beperformed in stages if desired prior to adding the substitute signals.

FIG. 1(b) is a block diagram of a substitute signal architecture 10consistent with the present invention. The substitute signalarchitecture 10 is preferably implemented in a DWDM optical network. Asshown in FIG. 1, a substitute signal architecture 10 includes a controlcircuit 20, a plurality of lasers 25, a combining circuit 30, a coupler35 and an optical signal analyzer (OSA) 40. The control circuit 20 iscoupled to the OSA 40 and receives signals from the OSA 40. The controlcircuit 20 also receives and sends signals and commands to and from anetwork management system (NMS) and to and from a pilot connection. Thepilot connection can couple a device, such as a laptop computer, to thecontrol circuit 20 to perform various functions for the substitutesignal architecture 10, including calibrating components in the systemand upgrading software.

The control circuit 20 is coupled to each of the lasers 25 by a controlline. The control circuit 20 controls the operation of each of thelasers 25 based upon the received signals and commands from the OSA 40,NMS and pilot connection. The operational control provided by thecontrol circuit 20 includes, for example, turning the lasers 25 on andoff, controlling the power levels of the lasers 25, maintaining thewavelength of the signals output from the lasers 25 at a particularsetting, and driving the modulation of the signals output from thelasers 25.

The control circuit 20 preferably includes a number of components toperform the operation control functions of the substitute signalarchitecture 10. These components include, for example, a microprocessoror CPU, a memory, such as a RAM or ROM, a network and pilot portinterface, a modulation control circuit, a power control circuit, and atemperature control circuit. The microprocessor executes software orfirmware in the memory to control the operation of the modulation, powerand temperature control circuits based on the received signals andcommands from the OSA 40, NMS and pilot connection. The modulation,power and temperature control circuits generate control signals outputon the control lines to control the operation of the lasers 25.

The lasers 25 receive control signals over the control lines from thecontrol circuit 20, and output a substitute signal at a particularwavelength. The lasers 25 may be implemented as Fabry-Perot lasers,distributed feedback lasers, directly modulated signal lasers orspectrally sliced ASE sources. Each laser 25 outputs a substitute signalat a different wavelength. The difference in wavelength output from eachlaser 25 may be approximately constant, i.e., there is an approximatelyconstant step between wavelengths of the lasers 25. Although only thelasers 25 are shown in FIG. 1, each laser 25 is part of a substitutesignal transmitter. Each substitute signal transmitter comprises arespective one of the lasers 25, as well as other circuitry forcontrolling the operation and modulation of the laser 25.

The modulation of each laser 25 can be controlled by a laser driver,such as an ILX laser driver. Modulation of the substitute signals isprovided primarily to avoid stimulated Brillouin scattering effects, butcan also be used for signaling purposes. The laser driver may, forexample, have a current modulation capability of up to 100 MHz, and morepreferably between 1 and 20 MHz, and preferably has a modulation depthof about 0 to 50%. Modulation frequencies may be chosen or formatted toavoid significant intermodulation signals within the informationbandwidth. The data modulated onto the signal output from the laser 25may be a random pseudo code, a sine wave or a square wave. The laserdriver may also include a temperature control, such as a thermoelectriccooling (TEC) circuit, and a back facet current monitoring circuit tocontrol the power of the laser 25, as well as a feedback control circuitto control the power of the laser 25.

The number of lasers 25 used in the substitute signal architecture 10may vary depending upon, for example, the bandwidth of the DWDM opticalnetwork and the number of signal channels. For example, for an opticalnetwork with a 100 nm bandwidth having between 256 and 384 signalchannels, the number of lasers 25 may be 32 or 48. Each lasercorresponds to a particular sub-band of the operational bandwidth of theoptical network. For example, for 256 signal channels and 32 lasers,each sub-band would include 8 contiguous signal channels, and for 384signal channels and 32 lasers, each sub-band would include 12 contiguoussignal channels. Those skilled in the art will appreciate that systemsaccording to the present invention may have fewer than 256 channels,e.g., 128 channels. The wavelength output from each substitute signallaser 25 may be approximately halfway between the shortest and longestwavelength of the signal channel in the associated sub-band.

Table I below shows an example of possible frequencies and correspondingwavelengths for a 32 laser substitute signal implementation.

TABLE I Laser Frequency Wavelength Number (GHz) (nm) 1 196.95 1522.15 2196.55 1525.13 3 196.15 1528.37 4 195.75 1531.51 5 195.35 1534.64 6194.95 1537.79 7 194.55 1540.95 8 194.15 1544.13 9 193.75 1547.32 10193.35 1550.52 11 192.95 1553.73 12 192.55 1556.96 13 192.15 1560.20 14191.75 1563.45 15 191.35 1566.72 16 190.95 1570.00 17 190.55 1573.30 18190.15 1576.61 19 189.75 1579.93 20 189.35 1583.27 21 188.95 1586.62 22188.55 1589.98 23 188.15 1593.37 24 187.75 1596.76 25 187.35 1600.17 26186.95 1603.59 27 186.55 1607.03 28 186.15 1610.39 29 185.75 1613.91 30185.35 1617.45 31 184.95 1620.86 32 184.55 1624.44

The substitute signals output from the lasers 25 are combined by thecombining circuit 30 into a multiplexed substitute signal. Combiningcircuit 30 can, for example, be implemented as a single unit, e.g., anarrayed waveguide (AWG). Various types of optical combining componentsmay be used in the combining circuit 30 to generate the multiplexedsubstitute signal. The optical combining components may be any one orcombination of 2×2 couplers, 4×4 couplers, wavelength combiners, arrayedwaveguides (AWGs), WDM MUXs, or wavelength interleavers. In particular,the use of multiple components for combining circuit 30 may be useful ifthe bandwidth associated with the substitute signal lasers 25 exceedsthe bandwidth of available (or commercially feasible) individualcomponents.

FIG. 2 is an example of an implementation of the combining circuit 30 ofthe substitute signal architecture 10 consistent with the presentinvention. As shown in FIG. 2, the combining circuit 30 includes twostages of combining. A first stage includes two combiners 32, and asecond stage includes a single combiner 34. In one aspect of the presentinvention, the combiners 32 may be implemented as AWGs. For animplementation of the substitute signal architecture 10 with 32substitute signals, each AWG receives and combines 16 substitutesignals. The substitute signals received by one of the AWGs may be inthe C-band, while the substitute signals received by the other AWG maybe in the L-band. The insertion loss for each AWG is preferably lessthan 5 dB.

The combiner 34 is coupled to receive the combined signals output fromthe combiners 32. In one aspect of the present invention, the combiner34 may be implemented as an interleaver or 2×2 coupler. Given the widebandwidth of commercially available interleavers, the use of aninterleaver as a second stage combiner, rather than as a mechanism forinterspersing even and odd channels, may be useful. The output of thecombiner 34 corresponds to the multiplexed substitute signal. Theinsertion loss for the interleaver is preferably less than 2 dB.Although not shown in FIG. 2, an amplifier may be included in thecombining circuit 30 to compensate for any loss in signal power of themultiplexed substitute signal.

The multiplexed substitute signal output from the combining circuit 30is provided to a coupler 35. The coupler 35 may be implemented as a 1%coupler, such that 99% of the multiplexed substitute signal is providedto a combining circuit that combines the multiplexed substitute signalwith the signal channels of the DWDM optical network and 1% of themultiplexed substitute signal is provided to the OSA 40. The OSA 40measures the wavelength and peak power of each substitute signal of themultiplexed substitute signal. These measurements are provided to thecontrol circuit 20.

The substitute load architecture 10 provides loading of sub-bands thathave unused or failed signal channels. This loading of sub-bands istypically used when the DWDM optical network is first implemented, whenthe optical network is upgraded to add bandwidth or additional signalchannels, and when there are any signal channel failures. The substituteload architecture uses the signals and commands from the OSA 40, the NMSand the pilot connection to establish which substitute signals areneeded to load a sub-band, as well as to maintain the proper operationof each laser 25 providing the substitute signal.

FIG. 3 is a flow diagram for processing a system command in thesubstitute signal architecture 10, consistent with the presentinvention. As shown in FIG. 1, a system command is received by thesubstitute signal architecture 10 (step 310). The system command may beprovided by the NMS or through the pilot connection. For example, inresponse to detecting the failure of one or more signal channels ordetecting that one or more signal channels are unused, the NMS may senda system command to the control circuit 20 instructing the controlcircuit 20 to turn on the lasers 25 generating the substitute signalscorresponding to the sub-bands having the unused or failed signalchannels. Alternatively, the NMS may send a system command instructingthe control circuit 20 to turn off a laser 25 corresponding to asub-band that is “being lighted,” i.e., the unused signal channels ofthe sub-band are becoming used.

After receiving the system command, the control circuit 20 identifiesthe type of command (step 320). As described above, the system commandmay be to turn on or turn off one or more substitute signals dependingupon conditions of the signal channels and associated sub-bands. Othersystem commands may also be received, such as requests for power andwavelength measurements of the substitute signals.

Based on the identified system command, the control circuit 20 eitherturns on or turns off one or more of the substitute signals (step 330).The system command can be processed by a microprocessor or CPU in thecontrol circuit 20, which executes software or firmware resident in amemory of the control circuit 20. If the system command is to turn on asubstitute signal, the control circuit 20 identifies the laser 25corresponding to the substitute signal and provides a control signalover a control line to the laser 25 to turn on the laser 25. The controlcircuit 20 also provides a control signal to the laser to control thepower level of the signal output from the laser 25. The control signalsmay be received and effected by the laser driver of the substitutesignal transmitter. If the system command is to turn off a substitutesignal, the control circuit 20 identifies the laser 25 corresponding tothe substitute signal and provides a control signal over a control lineto the laser 25 to turn off the laser 25.

In addition to receiving system commands, such as from the NMS, tocontrol the operation of the lasers 25 of the substitute signaltransmitters, the substitute signal architecture 10 may also turn on orturn off substitute signals based on measured power and wavelengths ofthe signal channels. FIG. 4 is a flow diagram for identifying andcompensating for failed signal channels in the architecture of FIG. 1.As shown in FIG. 4, the wavelengths and power levels of each of thesignal channels are measured (step 410). These measurements can be madeby the OSA 40, which receives a portion of the signal channels from acoupler, such as a 1% coupler, not shown in FIG. 1. Alternatively, anOSA independent of OSA 40, or another type of optical signal measuringdevice, may be provided in the optical network to measure the power andwavelengths of the signal channels. The measurements are provided to thecontrol circuit 20 via a signal line between the OSA 40 and the controlcircuit 20. Alternatively, the measurements may be provided to the NMSfrom a measuring device in the optical network, and the NMS thenprovides the measurement information to the control circuit 20.

Based on the measured power and wavelengths of the signal channels, itis determined whether any signal channel has failed (step 420). Usingthe measurements, the control circuit 20 can determine if the signalpower for a particular wavelength is below a threshold indicating thatthe signal channel has failed. The measurement information is processedby the microprocessor or CPU in the control circuit 20 using software orfirmware stored in a memory of the control circuit 20 to determine anysignal channel failures.

In addition to determining which signal channels have failed, the numberof failed or unused signal channels in the sub-band of the failed signalchannels is also identified (step 430). For example, if each sub-bandhas eight signal channels, then the number of failed or unused signalchannels in a particular sub-band may be between one and eight.

For each signal channel determined to have failed, the substitute signalis turned on that corresponds to the sub-band of the failing signalchannel (step 440). The control circuit 20 generates a control signalbased on the measurements and resulting determinations and provides thecontrol signal to the appropriate substitute signal transmitter to turnon the laser 25 for the substitute signal. In some circumstances, thesubstitute signal may not be turned on if only a few of the signalchannels in the sub-band have failed. For example, if two or fewersignal channels have failed, the still operable signal channels mayprovide enough loading to obviate the need for the loading provided bythe substitute signal. The processing in the control circuit 20 cancompare the identified number of failed or unused signal channels in thesub-band to a threshold, and turn on the substitute signal for thesub-band only if the number exceeds the threshold. The value of thethreshold may vary depending upon the number of signal channels in asub-band and based on testing of loading conditions for a reduced numberof operable signal channels.

The power of the turned-on substitute signal is adjusted based on theidentified number of failed or unused channels in the sub-band (step450). If there are still operable signal channels in the sub-band, thenthe signal power of the substitute signal does not need to be as high aswhen all of the signal channels in the sub-band have failed or areunused. The control circuit 20 can adjust the signal power of thesubstitute signal to be equal to, less than or greater than theaggregate amount of signal power corresponding to the failed or unusedsignal channels in the sub-band. If all of the signal channels areunused or have failed, the signal power of the substitute signal can beset to be equal to, less than or greater than the total power of thesignal channels in the sub-band. The setting of the signal power of thesubstitute signal that is used may vary according to the optimumperformance of the optical network as determined by the performance forvarious combinations of failed and/or unused signal channels. Theoptimum performance may be determined through simulations or testing ofthe network.

When any of the substitute signals are in operation, the substitutesignal architecture 10 can monitor and adjust the operation of thesubstitute signals to maintain the proper loading of the opticalnetwork. FIG. 5 is a flow diagram for compensating for power andwavelength drift in the substitute signal architecture 10 consistentwith the present invention. As shown in FIG. 5, the wavelengths andpower levels of each of the substitute signals are measured (step 510).These measurements are preferably made by the OSA 40, which receives aportion of the substitute signals from the coupler 35 shown in FIG. 1.

The measurements of the wavelengths and power levels of the substitutesignals are then received by the control circuit 20 (step 520). Themeasurements are received via a signal line between the OSA 40 and thecontrol circuit 20. Alternatively, the measurements may be provided tothe NMS from a measuring device in the optical network, and the NMS thenprovides the measurement information to the control circuit 20.

For each substitute signal, the measured power is identified based uponthe received measurement data (step 530). The measured power for eachsubstitute signal is then compared to the power setting for thesubstitute signal (step 540). Based on this comparison, the power forthe substitute signal is adjusted (step 550). The power is adjusted ifthe comparison indicates that there is a power drift. The power driftoccurs if the measured power is outside of a tolerance level of thepower setting. For example, the tolerance level may be within 1% of thepower setting, although other values may be used. If the measured poweris outside of the tolerance level, then the control circuit 20 sends acontrol signal to the substitute signal transmitter of the associatedsubstitute signal to adjust the signal power output from the laser 25 tocompensate for the power drift.

Power drift, as well as other factors such as laser case temperature,may affect the wavelength of the substitute signal, resulting inwavelength drift. A process similar to the one described above forcompensating for power drift can be applied to compensate for wavelengthdrift of the substitute signals. As shown in FIG. 5, the measuredwavelength for each substitute signal is identified (step 560). Themeasured wavelength is then compared to the wavelength setting for eachsubstitute signal (step 570). Based upon this comparison, the wavelengthof the substitute signal may be adjusted (step 580). The wavelength isadjusted if the comparison indicates that there is a wavelength drift,which occurs if the measured wavelength is outside of a tolerance levelof the wavelength setting. For example, the tolerance level may bewithin 1% of the wavelength setting, although other values may be used.If the measured wavelength is outside of the tolerance level, then thecontrol circuit 20 sends a control signal to the substitute signaltransmitter of the associated substitute signal to adjust the wavelengthof the signal output from the laser 25 to compensate for the wavelengthdrift. One way to adjust the wavelength of the signal output from thelaser 25 is to adjust the temperature of the laser case. The temperatureadjustment can be effected with the temperature control circuitry in thecontrol circuit 20 and the TEC circuit of the laser driver of thesubstitute signal transmitter.

As described above, the use of substitute signal transmitters togenerate substitute signals can maintain proper loading of an opticalnetwork in the event of a failure of a signal channel. The substituteload architecture 10 can also compensate for failures of substitutetransmitters. FIG. 6 is a block diagram of a redundancy circuitconsistent with the present invention. As shown in FIG. 6, a redundancycircuit includes redundant lasers 45 in addition to the lasers 25 shownin FIG. 1. Although not shown, the redundant lasers 45 may each be apart of a redundant substitute signal transmitter. Alternatively, theredundant lasers 45 may share the substitute transmitter circuitry ofthe lasers 25.

In the redundancy circuit of FIG. 6, each redundant laser 45 isassociated with a respective pair of lasers 25. In other words, eachredundant laser 45 is used to compensate for the failure of one or bothof the pairs of lasers 25 to which it corresponds. The wavelength of theredundant laser 45 may be approximately halfway in between thewavelength of the pair of lasers 25 to which it corresponds. AlthoughFIG. 6 shows two lasers 25 for each redundant laser 45, other ratios oflasers 25 to redundant lasers 45 may also be used.

The detection of a failure of a laser 25, or more generally, the failureof the substitute signal transmitter of the laser 25 is similar to thedetection of the power drift, as described above. Like the detection ofthe power drift, the power of each substitute signal is measured by theOSA 40 and communicated to the control circuit 20. The control circuit20 processes the measured power data and evaluates if the substitutesignal has failed, such as by comparing the measured power to athreshold, such that if the measured power is below the threshold, thesubstitute signal is determined to have failed. Based on the detectionof the failed substitute signal, the control circuit 20 activates theredundant substitute signal transmitter corresponding to the failedsubstitute signal and adjusts the power of the redundant substitutetransmitter to compensate for the loading lost by the failed substitutesignal.

Table II below shows an example of possible frequencies andcorresponding wavelengths for a 16 redundant laser implementation forthe 32 laser implementation of Table I.

TABLE II Redundant Laser Frequency Wavelength Number (GHz) (nm) 1 196.751523.66 2 195.95 1529.94 3 195.15 1536.22 4 194.35 1542.54 5 193.551548.91 6 192.75 1555.34 7 191.95 1561.83 8 191.15 1568.36 9 190.351574.95 10 189.55 1581.60 11 188.75 1588.30 12 187.95 1595.06 13 187.151601.88 14 186.35 1608.76 15 185.55 1615.62 16 184.75 1622.57

The substitute signal architecture 10 may also be designed to compensatefor gain tilt, which can result from the Raman gain effect. Due to theRaman gain effect, shorter wavelengths provide gain to longerwavelengths. With respect to the substitute signals, the substitutesignals having the shorter wavelengths can provide gain to thesubstitute signals having the longer wavelengths, resulting in a gaintilt. To compensate for the gain tilt, an attenuation splice can beinserted between the lasers 25 and the combining circuit 30. Theattenuation splice alters the insertion loss, which adjusts for the gaintilt.

FIGS. 7(a)-10(b) depict simulation results which Applicants havegenerated to test substitute signal architectures consistent with thepresent invention. Therein, the ability of the above-described 32 lasersubstitute signal embodiment to successfully load unused channels in awideband, Raman amplified optical communication system is confirmed.FIG. 7(a) depicts system launch power as a function of the wavelengthspectrum of operation when all of the sub-bands are filled withsubstitute signals, e.g., at system start up before data channels areturned on. FIG. 7(b) depicts the gain of the simulated received signalas a function of wavelength, which indicates that the substitute signalloading results in relatively little gain excursion, i.e., less thanabout 1 dB. FIG. 8(a) illustrates a simulation wherein one sub-band istransmitted with 8 data signals, while the rest of the system is stillloaded with substitute signals. Note that the data signals aretransmitted more closely together at lower power. FIG. 8(b) shows thesimulated received signals, which indicate that the received datasignals conform nicely with the received substitute signals. Theseresults are confirmed by the simulation illustrated in FIGS. 9(a) and9(b) which show the transmission of 80 data signals between about 1580nm and 1620 nm. Lastly, the simulation for a fully populated system (nosubstitute signals) is provided as FIGS. 10(a) and 10(b).

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and modifications and variations are possible in light in theabove teachings or may be acquired from practice of the invention. Theembodiment was chosen and described in order to explain the principlesof the invention and as practical application to enable one skilled inthe art to utilize the invention in various embodiments and with variousmodifications suited to the particular use contemplated. It is intendedthat the scope of the invention be defined by the claims appended heretoand their equivalents.

For example, the sub-band/signal substitution strategies discussed abovemay also be used in the context of implementing switches between servicefibers and protection fibers such as shown in FIG. 11. If a problemarises with a particular signal channel, then the system may detect thatproblem and switch all of the signal channels from the service fiber tothe protection fiber. The system turns on the substitute (sometimesreferred to as “dummy”) signal associated with the switched sub-band forthe protection fiber and turns off the substitute signal associated withthe switched sub-band for the service fiber.

1. A method for controlling substitute signal transmission in awavelength division multiplexed (WDM) device having a plurality ofsub-bands, each sub-band including at least two signal channels whichcarry respective data signals, and a plurality of substitute signaltransmitters, each substitute signal transmitter generating a substitutesignal and corresponding to a respective one of the plurality ofsub-bands, comprising: identifying signal channels having apredetermined characteristic within each of the plurality of sub-bands;turning on a substitute signal transmitter if the sub-band correspondingto the substitute transmitter includes a predetermined number of signalchannels having said predetermined characteristic; combining the datasignals and the substitute signals into a WDM signal; transmitting theWDM signal over an optical transmission fiber; detecting a fault in asubstitute signal transmitter based on detected wavelengths and powerlevels of the substitute signals; and turning on a backup transmittercorresponding to the substitute signal transmitter in which the fault isdetected.
 2. A method according to claim 1, wherein said predeterminedcharacteristic is one or both of unused and inoperable.
 3. A methodaccording to claim 1, further comprising: detecting the wavelengths andpower levels of each data signal in the WDM signal.
 4. A methodaccording to claim 3, further comprising: determining if a signalchannel has said predetermined characteristic based on the detectedwavelengths and power levels of the data signals.
 5. A method accordingto claim 1, further comprising: adjusting the power of the turned-onsubstitute transmitter depending on the number of unused or inoperablesignal channels in the identified sub-band.
 6. A method according toclaim 1, wherein said predetermined number is one.
 7. A method accordingto claim 1, wherein the number of backup transmitters is less than thenumber of substitute signal transmitters.
 8. A method according to claim7, wherein each backup transmitter corresponds to a respective pair ofsubstitute signal transmitters.
 9. A method according to claim 8,further comprising: adjusting the power of the backup transmitter basedon whether a fault has been detected in one or both of the pair ofsubstitute signal transmitters corresponding to the backup transmitter.10. A method according to claim 1, wherein the number of signal channelsis at least 128 and the number of sub-bands is no more than
 48. 11. Amethod according to claim 1, further comprising: attenuating the powerof the substitute signals output from each of the substitute signaltransmitters.
 12. A method according to claim 11, wherein the amount ofpower attenuated for substitute signals at lower wavelengths is lessthan the amount of power attenuated for substitute signals at higherwavelengths.
 13. A method according to claim 1, wherein saidpredetermined number is more than two.
 14. A method according to claim1, wherein said predetermined number is determined based on a number ofchannels in a sub-band.
 15. A method for controlling substitute signaltransmission in a wavelength division multiplexed (WDM) device having aplurality of sub-bands, each sub-band including at least two signalchannels which carry respective data signals, and a plurality ofsubstitute signal transmitters, each substitute signal transmittergenerating a substitute signal and corresponding to a respective one ofthe plurality of sub-bands, comprising: identifying signal channelshaving a predetermined characteristic within each of the plurality ofsub-bands; turning on a substitute signal transmitter if the sub-bandcorresponding to the substitute transmitter includes a predeterminednumber of signal channels having said predetermined characteristic;combining the data signals and the substitute signals into a WDM signal;transmitting the WDM signal over an optical transmission fiber; and,determining if there is wavelength drift in a substitute signal outputfrom a substitute signal transmitter based on detected wavelengths andpower levels of the substitute signals.
 16. A method according to claim15, further comprising: adjusting a temperature of the substitute signaltransmitter to compensate for the determined wavelength drift.
 17. Amethod for controlling substitute signal transmissions comprising:receiving a command to adjust a power of a substitute signal associatewith a particular sub-band, wherein said sub-band includes at least twosignal channels; adjusting said power based on said command, detecting afault in a substitute signal transmitter based on detected wavelengthsand power levels of the substitute signals; and turning on a backuptransmitter corresponding to the substitute signal transmitter in whichthe fault is detected.
 18. A method according to claim 17, wherein thenumber of backup transmitters is less than the number of substitutesignal transmitters.
 19. A method according to claim 18, wherein eachbackup transmitter corresponds to a respective pair of substitute signaltransmitters.
 20. A method according to claim 19, further comprising:adjusting the power of the backup transmitter based on whether a faulthas been detected in one or both of the pair of substitute signaltransmitters corresponding to the backup transmitter.
 21. A methodaccording to claim 17, wherein the number of signal channels is at least128 and the number of sub-bands is no more than
 48. 22. A methodaccording to claim 17, wherein said command to adjust a power is acommand to turn said power on.
 23. A method according to claim 17,wherein said command to adjust a power is a command to turn said poweroff.
 24. A method according to claim 17, wherein said command to adjusta power includes a power level value.
 25. A method according to claim17, wherein said command is based on a measured power level of said atleast two signal channels.
 26. A method according to claim 17, furthercomprising: turning on a predetermined number of signal channels in saidsub-band; and turning off a substitute signal, associated with saidsub-band, after said predetermined number of signal channels have beenturned on.
 27. The method of claim 26, wherein said sub-band includes atleast 8 signal channels and said predetermined number is greater than 2.28. A method according to claim 17, further comprising: detecting aproblem with a signal channel in a sub-band of a service fiber;switching all of the signal channels associated with said sub-band ofsaid service fiber to a protection fiber; and turning on a substitutesignal associated with said sub-band of said service fiber.
 29. Themethod of claim 28, wherein said sub-band includes at least two signalchannels.
 30. The method of claim 28, further comprising the step of:turning off a substitute signal for the corresponding sub-band in saidservice fiber.
 31. A method for controlling substitute signaltransmissions comprising: receiving a command to adjust a power of asubstitute signal associated with a particular sub-band, wherein saidsub-band includes at least two signal channels; adjusting said powerbased on said command; and, determining if there is wavelength drift ina substitute signal output from a substitute signal transmitter based ondetected wavelengths and power levels of the substitute signals.
 32. Amethod according to claim 31, further comprising: adjusting atemperature of the substitute signal transmitter to compensate for thedetermined wavelength drift.